For example, a general vacuum booster may be configured such that, a booster piston, which divides an interior of a booster shell into a front vacuum chamber which connects to a vacuum source and a rear operation chamber, is accommodated in the booster shell (see JP-H06-057527-B). To this booster piston, a valve cylinder which contains an input rod and a control valve which controls the communication of the operation chamber with the vacuum chamber and the atmosphere as the input rod moves back and forth may be continuously connected. A bearing bush made of an elastic material may be mounted in a bearing cylinder which is provided on a rear wall of the booster shell so as to support slidably an outer circumferential surface of the valve cylinder. A grease reservoir which holds lubrication grease may be provided in an inner circumferential surface of the bearing bush, and a seal lip, which is thinner than the bearing bush, may be provided at a rear end of the bearing bush continuously and integrally therewith so as to be brought into elastic close contact with the outer circumferential surface of the valve cylinder.
In the above-mentioned vacuum booster, the grease reservoir which holds grease is formed in the inner circumferential surface of the elastic bearing bush which slidably supports the outer circumferential surface of the valve cylinder so as to achieve the lubrication of the sliding portion between the bearing bush and the valve cylinder. However, since no grease reservoir exists in an inner circumference of the seal lip, when the valve cylinder slides back and forth, especially, when it slides forwards, a distal end portion of the thin seal lip is taken radially inwards by virtue of friction with the valve cylinder to thereby bring about a stick-slip phenomenon, whereby there may be caused situations in which not only is the smooth sliding of the valve cylinder prevented but also abnormal noise is produced.